Citing from Google‘s recent email on account inactivity: ”After a Google Account is deleted, the Gmail address for the deleted account cannot be used again when creating a new Google Account“
However, I do occasionally receive some emails on my fastmail user address which are not my own, which may indicate that Fastmail user addresses become available after unsubscribing.
The emails are from Steam, with 2FA login codes; I assume they only send those with prior email verification. With only the email address it is apparently not possible to contact Steam and tell them this is not my account (you need to login to contact support, or know a username). The danger here is that many websites (not just Steam) assume an email address is "forever", and the same address can not be used by anybody else.
https://www.fastmail.help/hc/en-us/articles/1500000279581-Ex...
> Once an account has been deleted, the username and any addresses on the account can be re-used.
https://www.fastmail.help/hc/en-us/articles/1500000277342-Ca...
And if you think verification does it, someone used their phone number and my email to register to Uber. I'd receive all their invoices and stuff. I couldn't log in because that needed phone verification. And it took quite a few rounds of Uber tech support to get them to remove my email.
They'll have to tackle email exhaustion eventually, and so far they don't seem interested in add new alternative domains.